Recently, a variety of image composition methods have been developed. Moreover, a variety of cameras that operate under a variety of settings (e.g., exposure amount and exposure time) are available, and those cameras acquire images with a variety of sensitivity characteristics for a wavelength band. Moreover, even cameras with the same settings acquire substantially different images depending on whether or not to light the flash during photography.
Moreover, cameras using sensors suited for photographing a variety of target objects are widespread. For example, a monitoring camera using a visible light sensor is widespread in order to monitor a person and other such objects. Meanwhile, a camera using a non-visible light sensor, for example, a near-infrared camera or a far-infrared camera, is also widespread for night-time monitoring. Moreover, among other cameras, a near-ultraviolet camera is also commercially available. Alternatively, among other cameras, a device configured to image a wavelength that is longer than a wavelength region of visible light, for example, a terahertz wave and a radio wave, is also commercially available.
In view of the above-mentioned background, in order to easily analyze input images acquired by a variety of sensors, it is required to improve an output image to an image that is suitable for a user. In Non Patent Document 1, there is disclosed, as image processing that is useful for such application, a method of performing processing based on gradient information (difference value between adjacent pixels) of an image.
In the method disclosed in Non Patent Document 1, as described in detail below with reference to FIG. 1, a target gradient is calculated based on a reference image group, and pixel values of an input image are renewed in such a manner that the target gradient and a gradient of an output image match each other to obtain the output image.